1. Field of the Invention
The present invention relates to a substrate for a magnetic recording medium and a magnetic recording medium using the substrate. Particularly, the present invention relates to a substrate for a high recording density magnetic recording medium such as a fixed type thin film magnetic recording disk used for information industries.
2. Discussion of Background
Recently, a fixed magnetic disk device is widely used as an outer recording device for an information processing device such as a computer. A magnetic disk in the fixed magnetic disk device is prepared generally by forming a NiP layer on the surface of a magneticless substrate comprising an aluminum alloy, applying a smoothing treatment, a texturing treatment or the like thereon, and then forming a magneticless metal underlayer, a magnetic layer, a protective layer, a lubricant layer and the like in order thereon.
In the magnetic disk device, a recording-reproducing head moves at a predetermined flying height on a magnetic recording medium, but recently the flying height became smaller and smaller in proportion to a rapid increase in a surface recording density of a magnetic recording medium. Further, a magnetic disk device tends to be miniaturized and lightened rapidly, and it is necessary to make a surface roughness of a magnetic recording medium much smaller in order to be applicable to this tendency, and actually the medium surface roughness Ra is reduced to a level of a few angstrom (.ANG.).
Also, in order to be applicable to a portable type fixed magnetic disk device, a shock resistance of a magnetic disk is required to be such a high value as 400G to 800G, and therefore a conventional substrate comprising an aluminum alloy tends to become unsatisfactory in shock resistance.
Consequently, in view of shock resistance and surface smoothness, a glass substrate capable of providing a very small surface roughness and excellent in mechanical strength tends to be used in place of an aluminum alloy substrate.
In case of an aluminum alloy substrate having a NiP layer formed, concentric texturing is applied on the surface toward the periphery of the substrate by grinding in most cases. This object is mainly to make an abrasive property between a recording-reproducing head and a magnetic recording medium satisfactory and to secure a satisfactory durability.
Also, recently, since a flying height of a head during operation of a magnetic disk device is remarkably small, texturing by laser beam is applied to CSS zone only in place of texturing by grinding. Thus, formation of protrusions by laser beam is tried. (see JP-A-8-129749).
However, since a glass substrate is generally transparent and is poor in absorptivity of laser and is hardly meltable, it is difficult to uniformly form protrusions of a specific shape as compared with a case of irradiating a NiP layer with laser beam.
Further, it is not easy to apply mechanical texturing to a hard glass substrate, and it is difficult to carry out uniform texturing in a case of glass ceramics since a crystallizing part and an amorphous part are different from each other in hardness.
Therefore, in order to apply laser texturing technique to a glass substrate, it is preferable to previously form a NiP layer on the substrate. For example, it is proposed in JP-A-61-54018 that NiP is formed on a glass substrate by electroless plating.
However, heretofore, there was a problem that adhesion between glass and NiP was poor. Therefore, in order to improve the adhesion between a glass substrate and a NiP electroless-plated layer, there has been proposed a method for mechanically or chemically roughening the surface of a glass substrate or a method for carrying out pretreatment of electroless plating.
However, smoothness and adhesion between a glass substrate and a NiP layer have been still unsatisfactory even by carrying out any of these method.
The present invention has been made in order to solve these problems, and its object is to provide a magnetic recording substrate comprising glass having a excellent adhesion to a NiP layer, and also to provide a magnetic recording medium having a high shock resistance and a satisfactory surface smoothness and stably achieving a low flying height of a head.